Adjusting proxy related params for global access

I've exposed the http_proxy* parameters to the global scope in this PR.  This may already be in the works with the stdmod refactoring going on, but I came across a use case this week.  It seems like a proxy is a good use case for a common parameter in the global scope that could be grabbed by any of the modules that could use a proxy.  I had written a yum.conf template to allow certain servers which require a proxy to use these top scope params for the yum proxy.  It seems the puppet-yum module doesn't have a local http_proxy_* param, but I could submit a patch there as well..  I've just run into a case where the same logic could be applied to Puppet

I have also submitted a feature request to TheForeman to see if they would look at allowing Subnet definitions to have parameters like Domains.  It would seem to make sense that proxy parameters might be specific to a subnet or group of subnets and this would be a great way to apply a different top scope parameter values dynamically across the landscape.